The present invention relates generally to wireless radio telecommunication systems, more particularly, it relates to the use of adaptive handoff algorithms for mobiles operating in CDMA systems.
Code division multiple access (CDMA) telecommunication systems, especially those designated as the so-called third generation wideband systems, such as CDMA (WCDMA) for example, were designed to provide access services that include circuit switched and packet switched data services having substantially improved bit rate performance. Such high bit-rate performance supported by third generation wideband systems gives rise to a multitude of services such as wireless multimedia and video, wireless data services such as simultaneous voice, and enhanced internet services.
The operating requirements of the various services may be considerably different thereby demanding correspondingly different performance levels from the system. By way of example, the characteristics of a circuit switched voice call typically are delay sensitive (i.e. no excessive breaks in the conversation which disrupts flow) high mobility, relatively short call duration, and symmetrical service (i.e. same amount of data is sent in both uplink and downlink directions). In contrast, packet based services such as internet web browsing are typically characterized by being delay insensitive (errant packets may be retransmitted), low mobility, relatively long call duration, and asymmetrical service i.e. downlink direction is typically more heavily used when loading web pages, for example. There are numerous factors that may affect the performance of various services operating within a CDMA system. One such factor that is known to affect the performance of typical services is mobile station handoffs.
FIG. 1 illustrates a basic cellular telecommunication system having a radio network controller (RNC) linked to a network of base stations by a series of digital transmission links 115. The base stations are geographically dispersed to form an area of coverage for the system. Each base station (BS) is designated to cover a specified area, known as a cell, in which t two way radio communication connection can take place between a mobile station and the BS in the associated cell. The boundary between the cells is indicated by line 110. In this simplified exemplary depiction, only two base stations are shown but in practice, a substantial multiplicity of base stations will form the junctional coverage area for the system. It is understood by those skilled in the art that other components and devices are typically included in the system that are not shown in the exemplary illustration. In general, as the MS moves throughout the network, communications are maintained with the network by transferring the connection to a neighboring base station in an event referred to as a handoff. For simplicity, the term mobile station will henceforth be referred to as the mobile.
In telecommunication systems operating in accordance with code division multiple access (CDMA), macro diversity is typically employed where a mobile simultaneously communicates with more than one BS prior to a handoff from an originating BS to a neighboring BS. This is referred to in the art as xe2x80x9csoft handoffxe2x80x9d in that the mobile will commence communication with the neighboring BS before terminating communication with the originating BS. This xe2x80x9cmake before breakxe2x80x9d procedure is made possible by operating all traffic on a common spread spectrum waveform frequency. A variant of the soft handoff is what is referred to as xe2x80x9csofter handoffxe2x80x9d in which the mobile simultaneously communicates with multiple sectors of the same BS. There are several advantages associated with soft handoffs such as reduced risk of dropped calls, no interruption in speech upon handoff, and increased gain in downlink signal-to-noise ratio. Another important advantage of soft handoff/softer handoff is that of macro diversity during mobility i.e. greater protection from log normal and multi-path fading since, on average, the convergence from the effects of fading or multi-paths do not occur at the same time.
Another type of handoff that occurs in CDMA systems is a xe2x80x9chard handoff.xe2x80x9d A hard handoff is a handoff that typically takes place, for example, between two channels or when the base stations are not suitably synchronized for a soft handoff. This type of handoff is often characterized as xe2x80x9cbreak before makexe2x80x9d since communication on a first frequency is terminated before communication is established on a second frequency. Hard handoffs occurring within the same cell are referred to as intra-cell hard handoffs and those occurring between cells are referred to as inter-cell hard handoffs. Hard handoffs typically occur in situations where vendor equipment limitations preclude performing soft handoffs such as, for example, layer changes for moving mobiles, mode switches e.g. in dual mode systems, switching between operator networks, and resource allocation issues that require hard handoffs.
In an exemplary CDMA system, handoff decisions are typically based on the detection by the mobile of the signal strength of pilot signals transmitted by neighboring base stations. The pilot signals are distinguished by a pseudonoise sequence (PN) such that the mobile is able to determine and allocate the base station within a distinct classification set. By way of example, the sets include an Active Set which is a set of base stations that the mobile is actively communicating with, a Candidate Set which is a set of base stations that have pilot strengths that are sufficient for communications based on system parameters set by the base station, and a Neighbor Set which is a set of base stations in the area that have a pilot strength indicating the potential for sufficient communication with the mobile. However, those skilled in the art will appreciate that the sets referred to and their functions are referenced by the CDMA standard known as IS-95 but that they have analogous counterparts with similar functions in other CDMA standards which may be identified differently. The base station""s classification within a set may be changed in accordance with, e.g., the received pilot signal strength by the mobile. Handoff decisions are then made by the system controller which are typically base its decision, at least in part, on the reported pilot signal strength and other criteria.
In the prior art, handoffs, that occur during voice and data services, are typically based on handoff algorithms that are unchanging and without regard to the impact on service performance. Accordingly, it is a object of the present invention to provide technique for utilizing handoff algorithms that adapt to the type of service requested.
To achieve the foregoing and other objectives in accordance with the purpose of the invention, an adaptive handoff algorithm governing mobile handoffs between sectors of the same or other base stations within a CDMA system is disclosed. In an embodiment of the invention, the handoff algorithm, using certain quality measures, is implemented in response to the particular type of service requested by the mobile. Handoffs governed by the handoff algorithm tend to lead to desirable operating environment for the mobile. By way of example, when a service such as a voice call is requested, the handoff algorithm uses a quality measure that takes into account the pathloss and uplink interference. This tends to induce handoffs that result the mobile connecting to a base station where output power of the mobile is minimized. This situation is desirable for conserving battery capacity of the mobile. In a further example, if the mobile is using a data intensive application such as internet browsing, the handoff algorithm uses a quality measure that induces the mobile to handoff to sectors that result in reduced downlink output power levels by the base stations. This tends to decrease the interference levels in the cell, thereby improving the chances of receiving the transmitted packets correctly and thus reducing the need for repeat transmissions.